This invention relates to semiconductor lasers with a structural design fabricated after device growth utilizing impurity induced disordering (IID). The present invention represents an improved semiconductor laser of the single or multiple emitter type utilizing impurity induced disordering (IID) techniques in regions not specifying, utilizing or otherwise requiring a quantum well feature.
Single emitter lasers generally of the III-V material regime, e.g., GaAs/GaAlAs, have a designed higher refractive index cavity which is formed between laterally adjacent regions having regions of comparatively lower refractive index. It is known to produce such optical cavities by means of nonplanar growth mechanisms, such as a channel or mesa in the laser substrate or by means of diffusion disordering as exemplified in U.S Pat No. 4,378,255 to Holonyak. As taught in this patent, a semiconductor structure containing a quantum well feature such as a multiple quantum well, undergoes compositional disordering due to impurity diffusion. Diffusion of an impurity into spatially separated regions of the quantum well feature will cause an intermixing of Al and Ga in the well feature so that the average refractive index through the region of these layers subjected to disordering by diffusion will have a lower index of refraction compared to undisordered regions including the central region between the designated spatially separated regions. Thus, the central region may be utilized as an optical waveguide cavity under lasing conditions.
Phased array semiconductor lasers comprise a plurality of closed coupled or spaced emitters on the same integral structure or substrate. Examples of such phased array lasers are illustrated in U.S. Pat. No. 4,255,717, now U.S. Pat. No. Re. 31,806, and in an article of William Streifer et al, entitled "Phased Array Diode Lasers", published in the June 1984 Issue of Laser Focus/Electro-Optics. The emitters of such a laser are represented by the periodically spaced current confinement means, e.g., stripes, for current pumping and establishment of spaced optical cavities in the active region of the structure. The current confinement means may be interconnected or closely spaced to a degree that the optical mode established in each of the lasing cavities below a respective current confinement means couples to its neighboring optical modes, i.e., the evanescent wave overlaps into adjacent optical lasing cavities. The array of optical fields produced become locked in phase, and, if the phase difference between adjacent current confinement means is zero, the lateral radiation pattern in the far field will comprise a single lobe. However, as explained in the above mentioned article, the phased array laser does not operate in a single mode but rather generally operate with two or more lobes in the far field pattern. The phase relationship between adjacent optical modes is not under independent control and the phases will adjust themselves in a manner toward minimizing laser threshold current. In most cases, it appears that the lasing mode favored is a supermode wherein the optical field between adjacent optical emitters passes through zero. This is because in most real refractive index lasers as well as many gain guided lasers, pumping is reduced at locations between the laser emitters requiring overall reduced current pumping.
Phased array lasers have high utility due to their high power output. It is preferred that the power be concentrated in a single lobe, i.e., in the 1st or fundamental supermode. The reason is that a substantial majority of laser applications require power in a single far field lobe. If lasing is experienced in more than one lobe, measures are taken to diminish or otherwise attempt to eliminate or block off the other operating lobes in the far field pattern.
Recently, there has been much discussion relating to phase locked array lasers or phased array lasers where efforts have been established to discriminate among the supermodes and provide fundamental supermode selection. One such suggestion was at the IEEE 9th Conference in Brazil, July, 1984 wherein J. Katz et al presented a talk on supermode discrimination by controlling lateral gain distribution along the plane of the lasing elements by incorporating a separate contact to each laser array element and tailoring the currents through the array laser elements. The abstract for the talk is found in the Proceedings of the Conference at pages 94 and 95 entitled "Supermode Discrimination in Phase-Locked Arrays of Semiconductor Laser Arrays".
More recently in the articles of Twu et al entitled "High Power Coupled Ridge Waveguide Semiconductor Laser Arrays", Applied Physics Letters, Vol. 45(7), pp. 709-711 (Oct. 1, 1984) and of S. Mukai et al entitled "Fundamental Mode Oscillation of Buried Ridge Waveguide Laser Array", Applied Physics Letters, Vol. 45(8), pp. 834-835 (Oct. 15, 1984). These articles suggest discrimination among the supermodes to obtain the single lobe fundamental supermode by employing index guided ridge waveguide structure wherein the laser elements are uniformly pumped with the optical field mainly confined to the ridge region of the structure while higher gain is experienced in the valley or coupling regions to induce inphase operation, i.e., 0.degree. phase, and promotion of fundamental supermode operation.
Further techniques to discriminate among supermodes are illustrated in U.S. patent application Ser. No. 736,040 filed May 20, 1985, entitled "Phased Array Semiconductor Laser With Preferred Emission in the Fundamental Supermode" and assigned to the same assignee herein. The techniques proposed in this application relate to the use of impurity induced disordering (IID) in a manner to enhance the amount of gain experienced in regions between adjacent optical cavities of lasing elements by spatially modulating the optical overlap of the optical field of each of the laser elements across the array to thereby favor the fundamental supermode over other potential modes. As previously mentioned, advances have been made in the art to better delineate the bandgap and refractive indices properties in a semiconductor device by disordering quantum well features which have been epitaxially deposited as part of a semiconductor device. An example of the foregoing is U.S. Pat. No. 4,378,255 wherein there is taught the technique of selectively disordering a multiple quantum structure or superlattice in a semiconductor device via a zinc diffusion through the quantum well structure thereby causing an upward shifting of the bandgap of the well regions of the quantum structure compared to regions of the multiple quantum well structure where disordering has not taken place. Such diffusion can be generally carried out in a temperature range of 500.degree. C.-600.degree. C., which is lower than the epigrowth temperature which is about 750.degree. C. Such disordering is also possible with other elements such as Si, Ge, Sn and S but at higher temperatures, e.g., about 675.degree. C. or above. Further, disordering is possible through implantation of elements acting as shallow or deep level impurities, such as, Se, Mg, Sn, O, S, Be, Te, Si, Mn, Zn, Cd, Sn or Cr followed by a high temperature anneal at temperatures optimium to each particular impurity, e.g., 500.degree. C.-900.degree. C. depending upon the type of impurity and best performed in an As environment. It also has been shown possible to disorder by implantation of III-V elements, such as A1. It has also been further shown possible to use a wide variety of elements to bring about disordering through implantation and annealing, e.g. the inert element, Kr, has been shown to induced disordering. In the case of impurity implant followed by an anneal, the anneal temperatures are relatively at higher temperatures compared to diffusion temperatures, e.g., above 800.degree. C. As use throughout this application, IID herein has reference to both the impurity diffusion technique or the implant/anneal technique both referenced above.